Basically, the principle of LED is to employ a characteristic of semiconductor to emit light. This is different from the conventional lighting apparatuses that lighting by discharging or heating, so the LED is called “cold light.” Compared with the conventional the light bulb or the fluorescent tube, the LED has advantages of high durability, long life, light, low power consumption, and mercury-free.
The basic structure of LED is a PN diode structure that comprises a P-type epitaxial layer, an N-type epitaxial layer, and an active layer there between. In general, a current is input by two pads on the epitaxial layers of the LED. The pads may be formed on the same side or opposite sides according to the material selection of substrate and epitaxial layer, as shown in FIG. 1A and FIG. 1B. For example of GaN-based LED, the common substrate is an unconductive sapphire (Al2O3) or a conductive SiC. FIG. 1A shows a structure of a GaN LED with a sapphire substrate 40. An N-type epitaxial layer 30, an active layer 20, and a P-type epitaxial layer 10 are on the substrate 40 in sequence. Pads 25 and 15 are formed on an exposed region of the N-type epitaxial layer 30 and the P-type epitaxial layer 10, respectively. FIG. 1B shows a structure of a GaN LED with a SiC substrate 50. An N-type epitaxial layer 30, an active layer 20, and a P-type epitaxial layer 10 are on the substrate 50 in sequence. Pads 15 and 25 are formed on an upper surface of the P-type epitaxial layer 10 and a lower surface of the substrate 50, respectively.
Due to the light-absorbing and light-covering problems of the pads 15 and 25, some manufacturers produce LEDs by Flip-Chip technology for improving the illumination of LEDs. As shown in FIG. 1C, the LED structure is a reverse structure of that shown in FIG. 1A. The structure has a P-type epitaxial layer 10, an active layer 20, an N-type epitaxial layer 30, and a substrate 40 in sequence from bottom to top, and light is emitted upwards from the side of substrate 40. The substrate 40 is transparent, so it has no problem of light-absorbing or light-covering. The flip-chip LED is generally adhered to a submount 60, and a reflecting layer is formed to reflect the downward light emitted by the LED upwards. Furthermore, the submount 60 may be made of a material (as metal) having good cooling effect to diffuse the heat of LED, so it is proper to operate under high current. Hence, the LED having flip-chip structure improves the illumination and cooling effect.
Although the illumination of flip-chip LED is substantially increased and the flip-chip LED is proper to operate under high current, the product yield thereof is not well. That is resulted from the conventional Flip-Chip technology is to connect the pads of LED with the submount by bonding, and the bonding yield is not well. Hence, the yield of flip-chip LED is not efficiently increased, and the product cost is higher.